Sort one sequence by O(n) in time and O(1) in space

[Wesley]

> > here is input sequence like a1,a2,...,an,b1,b2,...,bn ，the ax and bx always exist in pair. So, now, how to change the sequence to a1,b1,...,an,bn, with time complexity as O(n) and space as O(1).
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> The two halves of the list are already sorted, yes? 

[Wesley] No, not sorted yet..